The "how much money do I need to spend to make the annual fee worth it" really depends on how much value you place on the UR points. Let's conservatively say 1.5 cents.

For Chase Ink: You're getting 5x on the office supplies and telecom and 2x on the gas. Let's assume you average out to 4x. If you used a regular card, you'd get 1x, so you're getting a benefit of 3x. 3 * 1.5 cents = 4.5 cents per $ spent. So with a $95 annual fee, you need to spend $2,111 annually to make it worth it.

For Sapphire Preferred: Note that the 7% annual dividend is 7% of the POINTS you earned, not 7% of total spend. Valuing UR at 1.5 cents, you're getting an extra 0.105 cents per $ you spend throughout the year (.07 * 1.5). So you'd need to spend $90,500 to make the annual fee worth it.

Keep in mind that there are a lot of assumptions I've made. You have to do the math yourself with your own valuations and spending patterns to get a more solid answer.

But, important to remember is that there is a no annual fee version of the Ink card (called Classic) which still earns 5x at office supply and telecom and 2x on gas, but is capped at $25K instead of the $50K cap that you get on the Plus. And with the Ink Classic, the points are not transferable to airline and hotel partners. But if your spend is under $25K annually on the 5x categories and under $25K annually on gas, the best combo may be to switch to the Ink Classic (no AF) and keep the Sapphire Preferred. The Sapphire Preferred will make all your points transferable, and you can use it on all your non-bonus spending to get the small 7% bump, and no loss of points by downgrading from Ink Plus to Ink Classic.

The United card offers some perks, but no, it's not worth it to keep just for its regular earning. It gives the free checked bag, no forex fees (which one of your other cards will have), primary rental car coverage, and perhaps most importantly, a 10K miles bonus if you spend $25K in a calendar year. So if you don't care about the side perks and you aren't going to put $25K on it in a calendar year, ditch it. Note that for $25K of non-bonus spend (in a calendar year), it beats the Sapphire Preferred and Ink Plus.